This invention relates to the production of nanostructured (i.e., primary particles or crystalline size of less than 100 nm) lithium Iron phosphate (LiFePO4)-based powders as active cathode materials in Li-ion and Li rechargeable batteries.
There is an immediate need for reliable, safe, and non-toxic rechargeable batteries with high energy density, high power density, good shelf life, and low cost, for use in electric vehicle type applications. Such batteries can also be used in other commercial applications such as, wireless communication devices, camcorders and laptop computers. Rechargeable Li-based batteries, particularly rechargeable Li-ion batteries, are becoming the system of choice because of their overall good performance and high energy density. Presently, a majority of commercial Li-ion batteries use coarse LiCoO2 as cathode material; however, LiCoO2 is expensive as well as toxic, which renders it unsuitable for applications, such as electric and hybrid vehicles, that require batteries to be economical and environmentally friendly, along with good performance.
LiFePO4 has an ordered olivine type structure (olivine phase) and have recently been investigated as attractive cathode material because of its high theoretical capacity, 167-171 mAh/g, low cost and a lack of toxicity. FIG. 1 shows the olivine structure, where chains (along the c direction) of edge-sharing transition metal—octahedral are connected to one another by phosphate tetrahedra. These (FePO4) are connected to one another by octahedrally coordinated lithium atoms along the b axis ([A. K. Padhi, K. S. Nanjundaswamy and J. B., Goodenough, J. Electrochem. Soc., 144, 1188 (1977)]. Among all olivine LiMPO4 compounds (M=Co, Mn, Fe, Ni and V), LiFePO4 has been studied most extensively, since the demonstration by Padhi et al. that it is possible to fabricate electrochemically active LiFePO4 compounds. Later on, Yamada et al. [A. Yamada, S. C. Chung and K. Hinokuma, J. Electrochem. Soc., 148 (3), A224 (2001)] prepared coarse LiFePO4, and showed that it is possible to achieve a capacity of ≠160 mAh/g at a low current density. This data suggests that LiFePO4 cathode material has the potential to be a good candidate for Li-ion batteries. Additionally, both olivine-type materials, such as LiCoPO4 and LiMnPO4, are promising because they operate at higher voltage than LiFePO4 and can provide higher energy density. For example, in case of LiMnPO4, the redox potential for the Mn3+/Mn2+ couple is 4.1V, while it is 3.4 for Fe3+/Fe2+ couple in LiFePO4. (It is to be noted that the redox potential for the Co3+/Co2+ is 4.8 V). However, it has been shown that the capacity at 4.1 V is not achieved without Fe coexisting with Mn at the octahedral 4c sites [Padhi et al. and A. Yamada, Y. Kudo, and K. -Y. Liu, J. Electrochem. Soc., 148, A1153 (2001)]. Therefore, LiFexM1-xPO4 materials offer the potential of obtaining higher energy density Li-ion batteries than those using LiFePO4 cathodes.
Huang et al. [H. Huang, S. -C., Yin and L. F. Nazar, Electrochemical and Solid-State Letters, 4 (10), A170 (2001)] synthesized nanocomposites of LiFePO4 and conductive carbon by two different methods, which led to enhanced electrochemical accessibility of the Fe redox centers in this insulating material. In method A, a composite of phosphate with a carbon xerogel was formed from a resorcinol-formaldehyde precursor; in method B, surface oxidized carbon particles were used as nucleating agents for phosphate growth. They observed that electrochemical properties of powders prepared by method A were better because of the intimate contact of carbon with LiFePO4 particles. The resultant LiFePO4/C composite achieved 90% theoretical capacity at C/5, with good cyclability. In general, xerogels and aerogels have poor packing density, which will lead to low volumetric density of rechargeable Li-ion batteries. Chaloner-Gill et al. [U.S. Patent Publication No. US2002/0192137A1] describes the production of nanoscale and submicron particles of LiFePO4 and LiFe1-xMnxPO4 (0.4≦x≦0) by a laser pyrolysis method. However, laser pyrolysis methods are relatively expensive processes, and powders produced by such processes are thus not suitable for cost conscious applications, such as electric and hybrid vehicles.